Debilitating muscle weakness and fatigue are common side effects of chemotherapy in cancer patients, limiting treatment and increasing morbidity. In my dissertation research I found that healthy mice given doxorubicin, a chemotherapy drug, had decreased muscle force and an accelerated rate of fatigue. Mice deficient in tumor necrosis factor- receptor subtype 1 (TNFR1) were protected against the fall in muscle force. However, TNFR1 deficiency did not protect against doxorubicin-induced accelerated fatigue. These findings provide evidence that TNF at least partially mediates the decline in muscle function in response to doxorubicin and suggest persistent fatigue may be metabolic in origin. This project focuses on the metabolic, and more specifically mitochondrial aspects of muscle function. The combined effect of cancer and chemotherapy can compromise mitochondrial respiration and increase reactive oxygen species (ROS), potential mediators of the accelerated rate of fatigue. The central hypothesis of this project is that the combined effect of cancer and chemotherapy compromises mitochondrial respiratory control and increases ROS production, shifting the intracellular redox environment to a more oxidized state and decreasing muscle contractile function. To address this hypothesis all aims will include a comprehensive analysis of real-time mitochondrial function and cellular redox state conducted on permeabilized fiber bundles in conjunction with whole muscle fatigue analysis. This project will use tumor-bearing mice treated with the chemotherapeutic agent doxorubicin in Aims 1 and 2, followed by a translational study in breast cancer patients in Aim 3. Specific Aim 1 will determine the effects of cancer chemotherapy on skeletal muscle mitochondrial function, cellular redox state, and contractile function. We will determine the individual and combined effects of cancer/chemotherapy on multiple aspects of mitochondrial function in muscle. Specific Aim 2 will determine whether mitochondrial ROS production represents an underlying mechanism for cancer chemotherapy induced mitochondrial dysfunction and skeletal muscle fatigue. We will utilize pharmacological and transgenic mitochondrial-targeted antioxidant strategies to determine the role of mitochondrial ROS production in cancer and cancer chemotherapy-induced muscle dysfunction. Specific Aim 3 will determine the effects of cancer chemotherapy on skeletal muscle mitochondrial function in breast cancer patients receiving doxorubicin-based chemotherapy. This project will determine whether elevated mitochondrial ROS production is an underlying cause of skeletal muscle dysfunction, potentially establishing a mechanistic link to muscle dysfunction.